UPDATE 3-Explosion at Mexican oil refinery kills two

MEXICO CITY, July 30 (Reuters) - An explosion ripped
through Mexico's second-largest oil refinery on Saturday,
causing a massive fire and killing two workers, though
production was not affected, state oil monopoly Pemex said.

The explosion occurred at the 315,000 barrel-per-day Tula
refinery in central Mexico while the company was running a
trial of a visbreaker, a unit used to reduce viscosity when
distilling crude oil, a Pemex spokesman said.

The visbreaker, one of several at the refinery, recently
underwent maintenance work and Pemex workers were testing it on
Saturday to restart the unit, the spokesman said.

When a visbreaker fails, refineries end up with excessive
fuel oil inventories that could hamper operations.

One worker was taken to hospital with serious injuries
caused by the blaze, which was brought under control within an
hour, Pemex said in a statement.

"The accident occurred in a small, very localized area, and
so the refining production process was not affected," the
statement said. "Operations are continuing normally." Pemex
said it is investigating the cause of the accident.

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Factbox on Pemex refining operations [ID:nN09229970]

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The number of injured could be higher and a helicopter was
being used to airlift workers needing medical attention, said
Miguel Garcia, emergency services director for Hidalgo state.

Pemex employees were evacuated from the refinery but the
fire posed no risk to the surrounding area, Garcia said.

Clouds of black smoke were seen billowing out of the
refinery in pictures taken by residents near the blast and
posted on the social networking site Twitter.

Mexico imports about 40 percent of its gasoline because of
a lack of refining infrastructure in the oil producing nation.
The country imported 674,500 barrels per day of fuel in June,
according to Pemex data.

There are only six refineries in Mexico and serious
accidents in the past have lead to a spike in imports or
gasoline and diesel prices.

In September of last year an explosion killed one worker at
Pemex's Cadereyta refinery in northern Mexico, Mexico's third
largest oil processing facility.